The feature pragma is used to enable new syntax that would break Perl's backwards-compatibility with older releases of the language.
It's a lexical pragma,
like strict or warnings.

Currently the following new features are available: switch (adds a switch statement),
~~ (adds a Perl 6-like smart match operator),
say (adds a say built-in function),
and err (adds an err keyword).
Those features are described below.

Note that err low-precedence defined-or operator used to be enabled by default (although as a weak keyword,
meaning that any function would override it).
It's now only recognized when explicitly turned on (and is then a regular keyword).

Those features,
and the feature pragma itself,
have been contributed by Robin Houston.

This kind of match is called smart match, and it's also possible to use it outside of switch statements, via the new ~~ operator (enabled via the use feature '~~' directive). See "Smart matching in detail" in perlsyn.

Perl has now support for the CLONE_SKIP special subroutine. Like CLONE, CLONE_SKIP is called once per package; however, it is called just before cloning starts, and in the context of the parent thread. If it returns a true value, then no objects of that class will be cloned. See perlmod for details. (Contributed by Dave Mitchell.)

A new internal variable, ${^CHILD_ERROR_NATIVE}, gives the native status returned by the last pipe close, backtick command, successful call to wait() or waitpid(), or from the system() operator. See perlrun for details. (Contributed by Gisle Aas.)

The support for assertions, introduced in perl 5.9.0, has been improved. The syntax for the -A command-line switch has changed; it now accepts an optional module name, defaulting to assertions::activate. See assertions and perlrun. (Contributed by Salvador Fandiño García.)

assertions::compat, also available on CPAN, allows the use of assertions on perl versions prior to 5.9.0 (that is the first one to natively support them).

Math::BigInt::FastCalc is an XS-enabled, and thus faster, version of Math::BigInt::Calc.

Compress::Zlib is an interface to the zlib compression library. It comes with a bundled version of zlib, so having a working zlib is not a prerequisite to install it. It's used by Archive::Tar (see below).

IO::Zlib is an IO::-style interface to Compress::Zlib.

Archive::Tar is a module to manipulate tar archives.

Digest::SHA is a module used to calculate many types of SHA digests, has been included for SHA support in the CPAN module.

The interpreter internals now support a far more memory efficient form of inlineable constants. Storing a reference to a constant value in a symbol table is equivalent to a full typeglob referencing a constant subroutine, but using about 400 bytes less memory. This proxy constant subroutine is automatically upgraded to a real typeglob with subroutine if necessary. The approach taken is analogous to the existing space optimisation for subroutine stub declarations, which are stored as plain scalars in place of the full typeglob.

Several of the core modules have been converted to use this feature for their system dependent constants - as a result use POSIX; now takes about 200K less memory.

The new compilation flag PERL_DONT_CREATE_GVSV, introduced as an option in perl 5.8.8, is turned on by default in perl 5.9.3. It prevents perl from creating an empty scalar with every new typeglob. See perl589delta for details.

Parallel makes should work properly now, although there may still be problems if make test is instructed to run in parallel.

Building with Borland's compilers on Win32 should work more smoothly. In particular Steve Hay has worked to side step many warnings emitted by their compilers and at least one C compiler internal error.

Perl extensions on Windows now can be statically built into the Perl DLL, thanks to a work by Vadim Konovalov.

Configure will now detect clearenv and unsetenv, thanks to a patch from Alan Burlison. It will also probe for futimes (and use it internally if available), and whether sprintf correctly returns the length of the formatted string.

use strict "refs" was ignoring taking a hard reference in an argument to defined(), as in :

use strict "refs";
my $x = "foo";
if (defined $$x) {...}

This now correctly produces the run-time error Can't use string as a SCALAR ref while "strict refs" in use. (However, defined @$foo and defined %$foo are still allowed. Those constructs are discouraged anyway.)

Perl 5.9.2 introduced a change so that assignments of undef to a scalar, or of an empty list to an array or a hash, were optimised out. As this could cause problems when goto jumps were involved, this change was backed out.

Using the sprintf() function with some formats could lead to a buffer overflow in some specific cases. This has been fixed, along with several other bugs, notably in bounds checking.

In related fixes, it was possible for badly written code that did not follow the documentation of Sys::Syslog to have formatting vulnerabilities. Sys::Syslog has been changed to protect people from poor quality third party code.

Previously when running with warnings enabled globally via -w, selective disabling of specific warning categories would actually turn off all warnings. This is now fixed; now no warnings 'io'; will only turn off warnings in the io class. Previously it would erroneously turn off all warnings.

FindBin now works better with directories where access rights are more restrictive than usual.

Several memory leaks in ithreads were closed. Also, ithreads were made less memory-intensive.

Trailing spaces are now trimmed from $! and $^E.

Operations that require perl to read a process's list of groups, such as reads of $( and $), now dynamically allocate memory rather than using a fixed sized array. The fixed size array could cause C stack exhaustion on systems configured to use large numbers of groups.

PerlIO::scalar now works better with non-default $/ settings.

The x repetition operator is now able to operate on qw// lists. This used to raise a syntax error.

The debugger now traces correctly execution in eval("")uated code that contains #line directives.

The value of the open pragma is no longer ignored for three-argument opens.

Perl will now use the C library calls unsetenv and clearenv if present to delete keys from %ENV and delete %ENV entirely, thanks to a patch from Alan Burlison.

In general, the source code of perl has been refactored, tied up, and optimized in many places. Also, memory management and allocation has been improved in a couple of points.

Andy Lester supplied many improvements to determine which function parameters and local variables could actually be declared const to the C compiler. Steve Peters provided new *_set macros and reworked the core to use these rather than assigning to macros in LVALUE context.

Dave Mitchell improved the lexer debugging output under -DT.

A new file, mathoms.c, has been added. It contains functions that are no longer used in the perl core, but that remain available for binary or source compatibility reasons. However, those functions will not be compiled in if you add -DNO_MATHOMS in the compiler flags.

The AvFLAGS macro has been removed.

The av_*() functions, used to manipulate arrays, no longer accept null AV* parameters.

If you find what you think is a bug, you might check the articles recently posted to the comp.lang.perl.misc newsgroup and the perl bug database at http://bugs.perl.org/ . There may also be information at http://www.perl.org/ , the Perl Home Page.

If you believe you have an unreported bug, please run the perlbug program included with your release. Be sure to trim your bug down to a tiny but sufficient test case. Your bug report, along with the output of perl -V, will be sent off to perlbug@perl.org to be analysed by the Perl porting team.